Membrane filtration, or membrane technology for filtration, is a technique which is increasingly used in production of milk, such as milk with extended shelf life (ESL milk), or milk for manufacture of cheese (Cheese milk), etc. with the purpose of separating one or more components of the product from the rest. More particularly the present application will find its use in cross-flow filtration, involving guiding two parallel flows with a filter arrangement therebetween. The first flow comprises the raw material, referred to as ‘feed material’ containing one or more unwanted components, and the second flow, on the other side of the filter comprises a permeate (filtrate), i.e. the portion of the product which could pass through the filter. The part being left in the first flow after filtering is called the retentate (concentrate). For completeness this may be compared with dead-end filtration where a product is forced through a filter or sieve and the only way out is through the filter or sieve.
The process is driven by a pressure difference across the filter or membrane, and this pressure difference is referred to as transmembrane pressure (TMP).
In membrane filtration, there are different configurations of filtration modules that are used. A common feature for the basic layout is that there is an infeed and an outfeed on the retentate side, whereas there is only an outfeed on the permeate side Another common feature is that the filtration modules have a modular design, such that two or more modules may be coupled in series or in parallel to conform to the needs of a user.
A first type is the tubular design, which is a design similar to a tubular heat exchanger. The product/retentate is lead through small-diameter tubes in a bundled configuration, there is generally no contact between adjacent tubes. The small-diameter tubes comprise the membrane and the bundle of small-diameter tubes are arranged in a large-diameter tube or shell. The void between the outer perimeter of the small-diameter tubes and the shell provides the permeate side. The membranes may for example be formed from polymers or ceramics. One example of such configuration is disclosed in EP0333753.
Two other designs are the hollow-fibre designs and the spiral wound design, which will not be described in any detail in the present application.
The present application is primarily directed to filtration modules having a plate and frame design, a design which to some extent may be compared with the design of a plate heat exchanger. The feed material is driven along narrow channels in contact with one side of the membrane, and on the opposite side of the membrane the permeate may be collected. Several filtration modules may advantageously be arranged in a stack, in a parallel coupled configuration.
Two critical parameters in regard of microfiltration in general, and in microfiltration of food products in particular, are hygiene and capacity. Improvements in these parameters will have a direct impact on the capacity of a microfiltration device, for obvious reasons. The present application aims at providing an improvement on one or both these parameters. In regard of microfiltration performance the transmembrane pressure is a key parameter.